Casting mold and method of casting carbon-containing alloys



United States Patent CASTING MOLD AND IVIETHOD 0F CASTING CARBON-CONTAINING ALLOYS Hans Schneider, Winterthur, Switzerland, assignor to SulzenFreres, Societe Anonyme, Winterthur, Switzerland No Drawing. Filed Oct. 9, 1957, Ser. No. 689,061

Claims priority, application Switzerland Dec. 18, 1953 14 Claims. (Cl. 22-2165) This invention relates to casting molds and a method for the casting of metal alloys containing carbon.

In the casting of metal alloys having a carbon content,

CROSS REFERENCE a problem is created by the decarbonization of the surface of the casting during the cooling and solidification of the casting in the mold. While this decarbonization occurs in all types of casting it is particularly pronounced in casting by the destructible-pattern precision-casting method. When casting by this method a destructible pattern is first formed from wax, or some other combustible substance, with a low melting point. This pattern is coated with an aqueous ceramic paste. The pattern is melted and burned out to leave the green mold. The green mold is cured by firing it at a temperature that will drive out the water and solidify the mold. The molten metal is poured into the mold while the mold is still hot. The temperature of the mold before the metal is poured in may vary between 800 C. and 1000 C. If the molds are used for the production of casting alloys containing carbon, particularly high carbon steels, the portion of the casting near the surface has a tendency to decarbonize. This decarbonization is caused by the oxidation of the carbon near the casting surface through its coming in contact with oxygen which is present in and diffuses through the porous mold body. This decarbonization is disadvantageous because it creates a casting that is not homogeneous in structure. While decarbonization occurs in all types of castings made by commonlyaccepted methods, it is particularly extensive in complicated castings of large sizes. This is because the larger castings take longer to cool and the temperature remains above 800 C. for a comparatively long period. 800' C. is the critical temperature above which decarbonization occurs.

If such castings with decarbonized surfaces have to be hardened without prior mechanical treatment such as grinding, the decarbonized surface must first be recarbonized; this, however, possesses the following disadvantages. The metal at the surface of the casting will recrystallize, which causes stresses and strains to develop within the casting which often leads to fissures between that portion being recrystallized and the remainder of the casting.

. My invention provides a new method of making casting molds which prevents decarbonization by adding a combustible substance to the molding substance of which the mold body is made. The combustible substance, dispersed throughoutthe mold body, is provided to chemically react with the oxygen present in the mold body and diffusing through the mold body at temperatures higher than combustion temperature so as to prevent that oxygen from coming in contact with the carbon contained in the casting. The method of my invention comprises ice mixing a combustible substance such as an alkaline earth metal carbide or cyanide with a molding substance, applying said substance upon a destructible pattern, firing the mold, and casting a carbon-containing metal alloy therein. My invention also provides a mold made in accordance with the described method.

The method of my invention may be used, by way of example, in the destructible pattern method of precision casting of high carbon steel as follows:

A molding substance is made of a ground refractory substance. Calcium carbide is added to the molding substance, a wax pattern having an external configuration similar to the casting to be manufactured is encased in the ceramic paste. The wax pattern is melted out of the thusly formed green mold and the green mold is placed in a kiln for curing at a temperature approximately 800 C.900 C. The combustion temperature of the carbide is sufiiciently high so that it will not unite with oxygen at curing temperature. The calcium carbide content of the mold-body will also survive such a temperature without combining with oxygen. Immediately after the mold is taken out of the firing kiln the liquid steel, at a casting temperature of approximately 1500' C., is poured into the hot mold. Immediately the parts of the mold body adjacent the casting cavity are heated to a temperature slightly beneath the casting temperature. At this temperature the calcium carbide which is dispersed throughout the mold body will burn by combining with the oxygen present in the porous mold body thereby keeping that oxygen from coming into contact with the hot metal and causing the objectionable decarbonization of the metal near the surface of the casting.

I have found that alkaline earth metal cyanides may also be used with advantage as these compounds also burn at temperatures above the firing temperature of the mold to unite with the oxygen in the porous mold body when the liquid metal is poured into the mold. Alkaline earth carbides and cyanides are also easily mixed into a molding substance.

The amount of metal carbides and metal cyanides which can be used in accordance with the present invention to chemically react with the oxygen present in the mold body and difiusing through the mold body will 'be obvious to those persons skilled in the art. The minimum amount, of course, would be that which would react with substantially all of the oxygen to prevent decarbonization, and this would, of course, also vary with the size and density of the molding substance. It is not necessary to use amounts of metal carbides or metal cyanides in excess of that which will react with all of the oxygen. Naturally very large amounts which would interfere with the function of the molding substance should not be used. Generally the protective substance can be incorporated in amounts of 2 to 8% by weight of the molding mixture.

EXAMPLE 1 Calcium carbide A multiplicity of wax pattern were assembled together with a sprue in the form of a tree-like structure which serve as a positive pattern for the production of the casting mold. The sprue was similarly made of wax. The composite tree-like pattern thus constructed was then dipped into a liquid mold forming composition of a mixture prepared from finely comminuted quartz and partly hydrolyzed ethylgsilicat'aedissolved in alcohol as is well EXAMINER? l l l known in the art. The thin, non-self-supporting film deposited on the pattern was about 0.5-1 mm. in thickness. This coating was then dried and the pattern placed with the sprue downward upon a plate perforated for the subsequent discharge of the molten wax pattern. A cylindrical molders flask made of heat-resisting sheet metal was then placed over the entire coated pattern and the joint between the flask and the perforated plate was then sealed off with wax. A small amount of a liquid mixture of quartz sand and water glass to which an acid, such as hydrochloric acid, had been added for the purpose of accelerating the hydrolysis, was then added to the flask. This fluid mixture was permitted to stand until solidified to form a cover plate surrounding the sprue. The molders flask was then filled up with a dry, pourable, backing-up composition prepared as follows:

4 parts by weight of calcium carbide were added to 5 parts by weight of dry pulverized water glass and 91 parts by weight of molding sand quartz and the mixture thoroughly tumbled in a rotary drum. This dry, pourable, backing-up composition thus obtained, containing an inorganic bonding agent, is packed into the molders flask and firmly settled therein on a shaking table. The top face of the molders flask-was then covered with a slurry-like mixture of quartz sand and water glass to which an acid had been added to accelerate the hydrolysis to form a second cover plate the same as that formed above. After the cover had hardened the molders flask was transferred into a baking furnace and gradually raised to a baking temperature of 800-900 C. in a period of 6 to 8 hours. The calcium carbide content of the mold body survives such a temperature without combining with oxygen. During the baking operation the water glass liquefies by igneous fusion and eflects the desired bonding of the backing-up composition. If the wax pattern is to be recovered, the mold prior to being baked is transferred into a drying oven in which the wax is caused to run out at a slightly raised temperature. Immediately upon removal of the molders flask from the baking furnace, liquid steel at a casting temperature of approximately 1500" C. was poured into the hot mold. Immediately the parts of the mold body adjacent the casting cavity were heated to a temperature slightly beneath the casting temperature. At this temperature, the calcium carbide dispersed throughout the mold body burned by combining with the oxygen present in the porous mold body thereby keeping that oxygen from coming into contact with the hot metal and causing the objectionable decarbonization of the metal near the surface of the casting. After cooling the mold, the two covers were broken up and the mold material knocked out of the flask.

EXAMPLE 2 Barium cyanide The molding technique is the same as given in Example 1 with the exception that the barium cyanide may be used in an aqueous molding mixture used for backing up the coating formed on the wax pattern. This back-up mixture may have the following composition:

4% by weight of barium cyanide in an aqueous paste consisting of an aqueous slurry of quartz or chamotte, to which phosphoric acid had been added as the binding medium.

The castings made according to the method described are characterized by complete absence of decarbonization. Analyses made of samples of material taken from the surface of the castings after solidification show the same carbon content as material taken from the interior of the casting.

While the described method may be used in destructible-pattern precision-casting with great advantage, it is not limited to that type of casting but may be used with 4 advantage in ordinary steel casting with cold molds to prevent decarbonization.

This application is a continuation-in-part of my earlier application Serial No. 474,975, filed December 13, 1954,

now abandoned.

I claim:

1. The method of casting metal alloys having a carbon content comprising mixing a molding substance of ceramic material substantially free of water, adding an alkaline earth metal carbide to said substance, forming a refractory mold of said substance, and filling said mold with a carbon-containing molten-metal alloy.

2. The method of casting metal alloys having a carbon content comprising mixing a molding substance of ceramic material substantially free of water, adding barium carbide to said substance, forming a refractory mold of said substance, and filling said mold with a carbon-containing molten-metal alloy.

3. The method of casting metal alloys having a carbon content comprising mixing a molding substance of ceramic material substantially free of water, adding calcium carbide to said substance, forming a refractory mold of said substance, and filling said mold with a carbon-containing molten-metal alloy.

4. The method of casting metal alloys having a carbon content comprising mixing a molding substance of ceramic material, adding an alkaline earth metal cyanide to said substance, forming a refractory mold of said substance, and filling said mold with a carbon-containing moltenmetal alloy.

5. The method of casting metal alloys having a carbon content comprising mixing a molding substance of ceramic material, adding barium cyanide to said substance, forming a refractory mold of said substance, and filling said mold with a carbon-containing molten-metal alloy.

6. The method of casting metal alloys having a carbon content comprising mixing a molding substance of ceramic material, adding calcium cyanide to said substance, forming a refractory mold of said substance, and filling saidmold with a carbon-containing moltenmetal alloy.

7. An article of manufacture comprising a ceramic mold substantially free of water and having an alkaline earth metal carbide evenly dispersed throughout the mold body.

' 8. An article of manufacture comprising a ceramic mold having an alkaline earth metal cyanide evenly dispersed throughout the mold body.

9. An article of manufacture comprising a ceramic mold having barium cyanide evenly dispersed throughout the mold body.

10. An article of manufacture comprising a ceramic mold having calcium cyanide evenly dispersed throughout the mold body.

-11. An article of manufacture comprising a ceramic mold substantially free of water and having barium carbide evenly dispersed throughout the mold body.

12. An article of manufacture comprising a ceramic mold substantially free of water and having calcium carbide evenly dispersed throughout the mold body.

1=3. The method of casting metal alloys having a carbon content comprising mixing a molding substance of ceramic material, adding a member of the group consisting of an alkaline earth metal carbide and an alkaline earth metal cyanide to said substance, forming a'refractory mold of said substance, and filling said mold with a carbon-containing molten metal alloy, said substance being substantially free of water when an alkaline earth metal carbide is added to said substance.

14. An article of manufacture comprising a ceramic mold having a member of the group consisting of an alkaline earth metal carbide and an alkaline earth metal cyanide evenly dispersed throughout the mold body, said ceramic mold being substantially free of water when -5 having an alkaline earth metal carbide dispersed through- 1,706,858 Myers Mar. 26, 1929 out the mold body. 1,871,315 Gann Aug. 9, 1932 2,123,536 Long July 12, 1938 References Cited m the file of tlns patent 2,218,781 Baggett Oct 22, 1940 UNITED STATES PATENTS 5 2,435,121 Bean Ian. 27, 1947 820,099 Damhorst -2 May 8, 1906 1,308,243 Ianowsky July 1, 1919 FOREIGN PATENTS 1,650,700 Egler Nov. 29, 1927 299,854 Great Britain Feb. 3, 1930 

